Observations by NASA's Mars Reconnaissance Orbiter revealed light-colored mounds of a mineral deposited on a volcanic cone more than three billion years ago that may preserve evidence of one of the most the silica mounds on the flanks of a volcanic cone provide the best evidence yet found on Mars for an intact deposit from a hydrothermal environment — a steam fumarole, or hot spring believed to have provided habitats for some of Earth's earliest life forms.

NASA Researchers identifed the mineral as hydrated silica. The location of deposit probably made this a habitable zone," said J.R. Skok of Brown University. "If life did exist there, this would be a promising type of deposit to entomb evidence of it — a microbial mortuary."

No studies have yet determined whether Mars has ever supported life. The new results add to accumulating evidence that, at some times and in some places, Mars has had favorable environments for microbial life. This specific place would have been habitable when most of Mars was already dry and cold. Concentrations of hydrated silica have been identified on Mars previously, including a nearly pure patch found by NASA's Mars Exploration Rover Spirit in 2007.

However, none of those earlier findings were in such an intact setting as this one, and the setting adds evidence about the origin. "You have spectacular context for this deposit. It's right on the flank of a volcano. The setting remains essentially the same as it was when the silica was deposited," said Skok.

The small cone rises about 100 meters (100 yards) from the floor of a shallow bowl named Nili Patera. The patera — which is the floor of a volcanic caldera, spans about 50 kilometers (30 miles) in the Syrtis Major — discovered on the basis of data from Mars Global Surveyor to be a low-relief shield volcano in equatorial Mars.

Before the cone formed, free-flowing lava blanketed nearby plains. The collapse of an underground magma chamber from which lava had emanated created the bowl. Subsequent lava flows, still with a runny texture, coated the floor of Nili Patera. The cone grew from even later flows, apparently after evolution of the underground magma had thickened its texture so that the erupted lava would mound up.

"We can read a series of chapters in this history book and know that the cone grew from the last gasp of a giant volcanic system," said John Mustard, of Brown University. "The cooling and solidification of most of the magma concentrated its silica and water content."

Observations by cameras on the Mars Reconnaissance Orbiter revealed patches of bright deposits near the summit of the cone, fanning down its flank, and on flatter ground in the vicinity. Silica can be dissolved, transported and concentrated by hot water or steam. Hydrated silica identified by the spectrometer in uphill locations — confirmed by stereo imaging — indicates that hot springs or fumaroles fed by underground heating created these deposits. Silica deposits around hydrothermal vents in Iceland are among the best parallels on Earth.

The volcanic activity that built the cone in Nili Patera appears to have happened more recently than the 3.7-billion-year or greater age of Mars' potentially habitable early wet environments recorded in clay minerals identified from orbit. The habitable zone would have been within and along the conduits carrying the heated water.

It will be fascinating to see if future missions find silica deposits at the Olympus Mons — the tallest known volcano and mountain in our solar system located in the Tharsis region — an enormous volcanic plateau located on Mars' equator, at the western end of Valles Marineris.

The central edifice of this shield volcano stands 27 kilometers ( 88,580 ft) high above the surface of Mars-or three times the elevation of Mount Everest above sea level and 2.6 times the height of Mauna Kea above its base. It is 550 km in width, flanked by steep cliffs, and has a caldera complex that is 85 km long, 60 km wide, and up to 3 km deep with six overlapping pit craters. Its outer edge is defined by an escarpment up to 6 km tall; unique among the shield volcanoes of the Red Planet.

In 2004 the Express orbiter imaged old lava flows on the flanks of Olympus Mons. Based on crater size and frequency counts, the surface of this western scarp has been dated from 115 million years in age down to a region that is only 2 million years old — very recent in geological terms, suggesting that the mountain may yet have some ongoing volcanic activity.